Fixes the issue reported in PR #410 where `mo purge` fails to find
artifacts when `fd` returns empty results.
Changes:
- Implement MO_USE_FIND environment variable to force using find
- Improve fd fallback: check if fd output is empty (-s test)
- Add debug logging to show which tool is being used
- If fd returns no results, fallback to find automatically
This fixes the root cause where fd successfully runs (exit 0) but
finds nothing, preventing the find fallback from being triggered.
d8b396533d